G&P M16A2 Magic Front Lock Pin
-M4 receiver front pins can become loose over time but this one can be tightened to lock in place. (A much simple solution to a classic problem)
-The shaft of the pin is three cylindrical segments separated by angled edges all wrapping around a central screw.
-The pin slides into the receiver as normal but when the screw (hex / allen) is tightened the central segment is forced out of alignment sideways which locks the pin inside the receiver (until the screw is loosened).
-This pin is offered by G&P for both gas and AEGs.